How do u go back to windows after u changed it back to mac

Boot Using OPTION key

1. Restart the computer.
2. Immediately after the chime press and hold down the "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon for the Windows volume.
5. Click on the arrow button below the icon.

Do the same from Windows.

If you set the startup disk to a Mac OS X system, change it from the Startup Disk pane of System Preferences or restart with the Option key held down.

If you erased the Windows system, set it up again.

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